1. Field of the Invention
The present invention relates to an angular velocity sensor used, for example, in detecting an unintentional movement of a video camera, in operation detection in a virtual reality apparatus, in direction detection in a car navigation system, and the like.
2. Description of the Prior Art
An angular rate sensor oscillates an oscillator at a predetermined resonance frequency so as to detect with a piezoelectric element or the like a Coriolis force generated by affects of an angular velocity.
Conventionally, there are two methods for driving an oscillation gyro with a single piezoelectric element: a method using an externally-oscillated type drive circuit and a method using a self-oscillation type drive circuit. However, the method using the externally-oscillated type drive circuit has a problem that a difference is generated in the resonance frequency between oscillation frequency and the oscillator due to temperature characteristics of the oscillator, the piezoelectric element, the circuit or the like, which significantly lowers a Coriolis force detection sensitivity, and this type of sensor is not yet implemented in practice.
Currently, as shown in FIG. 1 and FIG. 2, an angular velocity sensor using a self-oscillation type drive circuit is used, employing a built-in oscillator in a phase oscillation circuit loop. In such an angular velocity sensor, as self-oscillation is obtained by the resonance frequency of the oscillator, the detection sensitivity is not much affected by the temperature characteristics, enabling to obtain a stable detection sensitivity in a wide range of temperature.
FIG. 1 shows an angular velocity sensor includes an oscillator 104 made from a constant-elastic metal oscillator 100 having a shape of a triangle pole whose faces has a first piezoelectric element 101 consisting of an electrode 101a and a piezoelectric body 101b, a second piezoelectric element 102 consisting of an electrode 102a and a piezoelectric body 102b, and a third piezoelectric element 103 consisting of an electrode 103a and a piezoelectric body 103b. Moreover, this angular velocity sensor includes: an amplifier 105 connected to the first piezoelectric element 101; a phase converter 106 connected to the amplifier 105; a differential amplifier 107 connected to the second piezoelectric element 102 and to the third piezoelectric element 103; a synchronization detector 108 connected to the differential amplifier 107; and a low pass filter 109 connected to the synchronization detector 108.
Such an angular velocity sensor using the oscillator of a triangle pole shape has currently the highest sensitivity and is widely used. In this angular velocity sensor, in order to cause self-oscillation, the second piezoelectric element 102 and the third piezoelectric element 103 detect an oscillation of the oscillator 104 as well as a Coriolis force generated in the oscillator 104. That is, in this angular velocity sensor, the same piezoelectric element serves for two functions: detecting an oscillation of the oscillator 104 for generating self-oscillation and detecting a Coriolis force. For this, in this angular velocity sensor, the voltage used for detecting a Coriolis force is limited by the drive voltage of the oscillator 104 and the power voltage, and it is impossible to sufficiently utilize the detection capability of the piezoelectric elements.
On the other hand, FIG. 2 shows an angular velocity sensor including an oscillator 115 made from a constant-elastic metal oscillator 110 having a shape of a square pole whose faces has a first piezoelectric element 111 consisting of an electrode 111a and a piezoelectric body 111b, a second piezoelectric element 112 consisting of an electrode 112a and a piezoelectric body 112b, a third piezoelectric element 113 consisting of an electrode 113a and a piezoelectric body 113b, and a fourth piezoelectric element consisting of an electrode 114a and a piezoelectric body 114b. Moreover, this angular velocity sensor includes: an amplifier 106 and a phase converter 116 connected to the first piezoelectric element 111 and to the second piezoelectric element 112; a differential amplifier 119 connected to the third piezoelectric element 113 and to the fourth piezoelectric element 114; a synchronization detector 119 connected to the differential amplifier 118; and a low pass filter 120 connected to the synchronization detector 119.
In the angular velocity sensor using such an oscillator 115 of a square pole shape, the first piezoelectric element 111 generates oscillation of the oscillator 115; the second piezoelectric element 112 detects oscillation of the oscillator 115 for generating self-oscillation; and the third piezoelectric element 113 and the fourth piezoelectric element 114 are used to detect a Coriolis force generated in the oscillator 115. That is, in this angular velocity sensor, each of the piezoelectric elements is used for a single function: the piezoelectric element for oscillating the oscillator 115, the piezoelectric element for detecting oscillation of the oscillator 114 for generating self-oscillation, and the piezoelectric elements for detecting a Coriolis force. Consequently, it is possible to utilize a sufficient capability of each of the piezoelectric elements.
However, this angular velocity sensor requires at least two piezoelectric elements for driving the oscillator and furthermore, at least four piezoelectric elements if those for detecting Coriolis force are included. This makes a disadvantage in production costs. Moreover, when such a plenty of piezoelectric elements are used, it becomes difficult to adjust the oscillation frequency of the oscillator 115. Thus, an angular velocity sensor using the oscillator 115 of a square pole shape has various problems and has not yet been implemented in practice.
As has thus far been described, the method using an externally-oscillated type drive circuit has a problem of sensitivity change due to temperature characteristics. This problem can be solved by a self-oscillation type drive circuit using a phase oscillation circuit, but the angular velocity sensor shown in FIG. 1 sacrifices sensitivity because the same piezoelectric elements are shared for driving the oscillator 104 and for detecting a Coriolis force. On the other hand, the angular velocity sensor as shown in FIG. 2 has the disadvantage in production costs and also has a problem that it is difficult to adjust the frequency, which disables to implement in practice.
Moreover, in the conventional angular velocity sensors as shown in FIG. 1 and FIG. 2, the circuits of the amplifiers 105 and 111, the phase converters 106 and 115 and the like are comparatively complicated, and it is desired to develop an angular velocity sensor having a simplified circuit.